A miranda system, a slipway system and a dock system are known as typical systems of the launch and recovery apparatus and method, which launch and recover a boat, such as a lifeboat, a rescue boat, or a high-speed surveillance and rescue boat.
The launch and recovery apparatus of the miranda system is known as a miranda davit, in which a davit member is mounted on a deck of a marine vessel, a cradle moves upward and downward on a track of the davit member, and they are used for launching and recovering the boat with the boat being held by the cradle (e.g., Japanese Patent Laid-Open Publications Nos. 56-25083, 61-184194 and 9-71292). Further, the launch and recovery apparatus of the slipway system has an arrangement in which the boat is launched or recovered along a slope provided in a stern part of a marine vessel. The apparatus of the miranda or slipway systems can perform such a launch and recovery of the boat only when the marine vessel stays in a substantially stationary condition.
On the other hand, the dock system is known as a system in which the boat is accommodated in the dock provided at the stern part of the marine vessel and so forth. In general, the launch and recovery apparatus of the dock system includes a stern gate which can open and close. The dock opens backward of the vessel when the gate opens. Since the seawater in the rear of the vessel flows into the dock through the stern opening, a water level necessary for maneuverability of the boat is kept in the dock. Therefore, the boat in the dock can move outboard through the stern opening when the gate opens, and the boat in the outboard area can enter the dock through the stern opening when the gate opens. According to such a launch and recovery apparatus, launch and recovery of the boat through the stern opening can be carried out during headway navigation of the marine vessel.
However, if the gate is opened during high-speed headway navigation, a suction effect expelling the water from the dock occurs, and therefore, the water level in the dock tends to lower. Simultaneously, a flow field in the rear of the vessel tends to rise relative to the water level in the dock, in connection with an affection of propeller race of the marine vessel. The boat has difficulty in entering the dock, owing to such descent of the water level in the dock and elevation of the rear flow field. Therefore, in the conventional dock system, the launch and recovery operation of the boat cannot be carried out during high-speed headway navigation of the vessel, but it is merely possible to perform the launch and recovery operation of the boat only in a condition of low-speed navigation of the vessel (during navigation at headway speeds up to 5 knots).
On the other hand, during low-speed headway navigation or stoppage state of the marine vessel, the water in the dock is kept at rest with respect to the vessel (mother ship), and therefore, rising of the rear flow field does not occur. However, a driving resistance of the boat approaching the dock rapidly decreases immediately after it enters the dock, and therefore, the boat approaching the dock tends to suddenly accelerate immediately after entering the dock, in connection with rapid reduction of the driving resistance. In fact, this results in, e.g., an excessive impact acting on the hull owing to impingement of the suddenly accelerated boat against the end wall of the dock, or difficulty in maneuvers of the boat in the dock. In order to avoid such a situation, it is necessary to promptly and properly control the thrust of the boat at the time of entry of the boat into the dock. However, this imposes a burdensome task on an operator of the boat.
Further, if the gate is opened in a high-wave state, especially in a following-wave state, the waves entering the dock impinge against the end wall of the dock, so that impacts act on the hull. Therefore, it is difficult to launch or recover the boat in such a sea state.
It is an object of the present invention to provide an apparatus and method of a dock system for launch and recovery of a boat stowed in a marine vessel, which can launch and recover the boat while the vessel makes headway at a high speed.
It is another object of the present invention to provide an apparatus and method of a dock system for launch and recovery of a boat stowed in a marine vessel, which can overcome a problem of difficulty in maneuvering of the boat entering the dock of the vessel in low headway speed navigation or stoppage condition; a problem on difficulty in maneuvering of the boat during launch or recovery operation under a following-wave state, and a problem of an impact on a hull or the like occurring in such launch or recovery operation, whereby launch and recovery operation of the boat is facilitated.